Card-type portable devices based on the PCMCIA (Personal Computer Memory Card International Association) standards are used as portable devices that have batteries built-in. If a card-type portable device is inserted in a personal computer, data are transferred from the personal computer to the card-type portable device. A power source then used for the operation of the card-type portable device is supplied from the personal computer side. Built-in batteries are used as a power source for the operation of the card-type portable device after the card-type portable device to which the data are transferred is separated from the personal computer. Accordingly, the card-type portable device requires use of a power switching circuit for selecting the source of power supply, an external power source (power source on the personal computer side) or built-in batteries.
FIGS. 6 and 7 individually show two prior art examples of the power switching circuit used in a portable device.
In a first example shown in FIG. 6, an external power source 601 (e.g., 5 V) is connected to a portable device body 613 through a switch 602 and a diode 611. Further, a built-in battery 604 (e.g., 3 V) is connected to the portable device body 613 through a diode 610.
In the case where the portable device is separated from the rest, the switch 601 is opened, so that the portable device body 613 is supplied with power from the built-in battery 604 through the diode 601. As this is done, the diode 611 prevents current from flowing out of the built-in battery 604.
When this switch is closed so that the external power source 601 is also connected to the portable device body 613 through the diode 611, on the other hand, the voltage of the external power source 601 is supplied to the portable device body 613 only in the case where the voltage of the external power source 601 is higher than the voltage of the built-in battery 604. At this time, the diode 610 prevents the external power source from charging the built-in battery 604.
In a second example shown in FIG. 7, a field-effect transistor (hereinafter referred to as FET) is used between a built-in battery and a portable device body. An external power source 701 (e.g., 5 V) is connected to a portable device body 713 through a switch 702 and a diode 711. On the other hand, a built-in battery 704 (e.g., 3 V) is connected to the portable device body 713 through a P-channel FET 710.
The source and drain of the P-channel FET 710 are connected to the portable device body 713 and the built-in battery 704, respectively. Further, the gate of the FET 710 is connected to the external power supply switch 702, and is also connected to the negative electrode of the built-in battery 704 through a bias resistor 709. The negative electrode of the built-in battery 704 is connected to the negative electrode of the external power source 701 and the negative-electrode-side terminal of the portable device body 713.
When the external power supply switch 702 is open, the drain of the FET 710 is connected to the positive electrode, 3 V, of the built-in battery 704, while the gate thereof is connect to the negative electrode, 0 V, of the built-in battery 704 through the bias resistor 709. Further, the source of the FET 710 is adjusted substantially to the drain voltage (3 V) by means of a drain-source parasitic diode. In consequence, the source-gate voltage is at -3 V. Accordingly, the FET 710 is turned on, so that the voltage (3 V) of the built-in battery 704 continues to be supplied to the portable device body 713. As this is done, the diode 711 prevents current from flowing out of the built-in battery 704.
When the external power supply switch 702 is closed, on the other hand, the gate and source of the FET 710 are adjusted to the voltage, 5 V, of the external power source 701, and the drain thereof to the voltage, 3 V, of the built-in battery 704. In consequence, the source-gate voltage is at 0 V. Accordingly, the FET 710 is turned off, so that the voltage of the built-in battery 704 is prevented from being supplied to the portable device body 713, and the voltage, 5 V, of the external power source 701 is supplied to the portable device body 713 through the diode 711. At this time, the parasitic diode of the off-state FET 710 prevents the built-in battery 704 from being charged with the voltage of the external power source 701.
In many conventional personal computers, 5 V is supplied to a PCMCIA card interface. In many cases, on the other hand, PCMCIA memory cards that are adapted to be inserted into personal computers have 3-V coin-type lithium batteries built-in. Thus, in a card-type portable device, the voltage of the external power source is higher than the voltage of the built-in battery, so that the aforesaid two prior art examples are applicable. While the external power source is being supplied, the built-in battery can be prevented from being dissipated wastefully.
In view of working power, it is advisable to use some card-type portable devices at 6 V with two 3-V coin-type lithium batteries that are connected in series. In this case, however, the voltage (5 V) of the external power source is lower than the voltage (6 V) of the built-in batteries, so that the card-type portable devices of this type cannot be used in combination with a conventional personal computer in which 5 V is supplied to the PCMCIA card interface.